1. Field of the Invention
The present invention relates to electronic read-out devices, and, more particularly, to an apparatus for providing frequency multiplexing of multiple flux locked loops in systems comprising an array of superconducting quantum interference devices (DC SQUIDs).
2. Description of the Prior Art
DC SQUIDs are small, cryogenically-cooled magnetic sensors that comprise a ring of superconducting material interrupted by two Josephson junctions. DC SQUIDs are designed to detect changes in magnetic flux, and, when suitably biased with a small DC current, will exhibit a magnetic flux sensitivity noise floor of approximately 1xc3x9710xe2x88x926xcfx860/ Hz for low temperature devices that operate at 4 degrees Kelvin (typically cooled by liquid Helium), and approximately 7xc3x9710xe2x88x926xcfx860/ Hz for high temperature devices that operate at 77 degrees Kelvin (typically cooled by liquid Nitrogen). Furthermore, DC SQUIDs exhibit a transfer function that converts magnetic flux into a periodic electrical output signal.
Although a single DC SQUID is sufficient for some applications, there are applications in which it is desirable to employ multiple DC SQUIDs. For example, in non-destructive test and evaluation applications a linear or rectangular array of DC SQUIDs using a sinusoidal excitation functions as a phased array in which the usable DC SQUID measurement directionality can be narrowed and lengthened to improve lateral resolution and increase depth of penetration. While it is possible to simply link multiple single-DC SQUID systems to form such an array, this arrangement is highly undesirable in terms of both redundant componentry and inefficient operation.
One problem arises due to the number of connections between the DC SQUIDs and their associated room temperature read-out electronics. In a typical single-DC SQUID system, there are at least two wire pairs or transmission lines connecting each DC SQUID to its associated room temperature electronics. Thus, for example, in an array formed of ten single-DC SQUID systems, there would be at least twenty of these connections. Such a large number of connections causes high heat transfer to the cryogenics, increases system complexity, and reduces reliability. Attempts at multiplexing DC SQUID arrays used time multiplexing with switches, which involves increased complexity and other problems, such as switching transients.
Another problem involves the amount of redundant componentryxe2x80x94each DC SQUID sensor requires its own complete flux locked loop (FLL) for providing feedback and maintaining a stable operating point.
For these reasons, magnetic measurement systems comprising arrays of DC SQUIDs are complex, costly, and inefficient.
The frequency multiplexed flux locked loop architecture of the present invention includes essential enabling technology which makes the operation of multi-DC SQUID systems more efficient, economical, and practical. The present invention comprises a system with continuous signals and no time switching devices and therefore none of the associated problems found in the prior art. The present invention makes use of the high bandwidth achieved by the read-out electronics disclosed in the copending patent application entitled xe2x80x9cA Fast Flux Locked Loopxe2x80x9d, Ser. No. 09/596,135, filed Jun. 16, 2000, and the copending patent application entitled xe2x80x9cRead-Out Electronics for DC SQUID Magnetic Measurementsxe2x80x9d, Ser. No. 09/596,190, filed Jun. 16, 2000.
Another novel feature of the present invention is that each radio-frequency (RF) flux locked loop (FLL) and its corresponding DC SQUID operate on a different flux modulation frequency (f1 through fN). This allows for a 1xc3x97N architecture which reduces from 2N to N+1 the number of required cable connections between the cryogenic DC SQUIDs and their associated room temperature read-out electronics. Thus, for example, a system comprising an array of ten DC SQUIDs, which previously would have required at least twenty cable connections, with their associated heat transfer and added complexity, now requires only eleven connections. Within practical limits, any number of DC SQUIDs can be connected together in a series array using the architecture of the present invention.
While the term 1xc3x97N implies linearity, this only applies electrically and does not mean that the physical DC SQUID array must be linear. Furthermore, the 1xc3x97N architecture can be paralleled in order to increase array size. The frequencies f1, f2, . . . fN can be repeated in M parallel systems without any interference between the 1xc3x97N subsystems. A rectangular array, or an extended linear array, can be easily constructed in this way. For example, a system comprising 150 DC SQUIDs can be arranged as a 15xc3x9710 array. This system would be composed of M=15 parallel copies of the 1xc3x97N architecture, each with N=10. Frequencies f1, f2, . . . fN would be the same for each parallel system so that only ten frequencies need be used in implementing the entire array. The total number of SQUID connections would be 15xc3x97(10+1)=165, which is 135 connections (45%) less than the 15xc3x97(2xc3x9710)=300 connections required for a non-multiplexed system.
Yet another novel feature of the present invention is that it reduces flux locked loop component redundancy by sharing certain components among the DC SQUIDs, thereby reducing complexity, cost, and size of the apparatus as a whole. Thus, while each DC SQUID still requires some dedicated, frequency-specific flux locked loop componentry, other non-frequency-specific flux locked loop component functions are performed by shared or common components.
These and other important aspects of the present invention are more fully described in the section entitled Detailed Description, below.